1. Field of the Invention
The present invention relates to a magneto-resistive effect (MR) element and particularly to a configuration of a spacer layer.
2. Description of the Related Art
Reproducing heads with high sensitivity and high output are in demand in conjunction with condensing of high recording density in hard disk drives (HDD). As an example of this type of reproducing head, a spin valve head has been developed. A spin valve head includes a nonmagnetic metal layer and a pair of ferromagnetic layers positioned on both sides of the nonmagnetic metal layer in a manner of contacting the nonmagnetic metal layer. The magnetization direction of one side of the ferromagnetic layers is pinned in one direction (hereinafter, this type of layer is referred to as a magnetization pinned layer), and the magnetization direction of the other side of the ferromagnetic layers freely rotates in response to an external magnetic field (hereinafter, this type of layer is referred to as a magnetization free layer). When an external magnetic field is applied, the relative angle of spins between the magnetization pinned layer and the magnetization free layer changes so that magneto-resistive change is realized. Typically, the magnetization direction of the magnetization pinned layer is pinned by utilizing exchange coupling force of an anti-ferromagnetic layer. In the present specification, a stack in which the above-described pair of ferromagnetic layers and a spacer layer are laminated is referred to as a magneto-resistive effect element (MR element).
On the other hand, in order to realize further condensing of high recording density, a reduction of a read gap (a space between upper and lower shield layers or a height of the MR element in a lamination direction) is required. However, when the read gap is reduced to approximately 20 nm, placing an anti-ferromagnetic layer within the read gap becomes difficult. Therefore, a configuration has been developed in which a pair of magnetization free layers is arranged on both sides of the spacer layer. Since no anti-ferromagnetic layer is needed with this configuration, it becomes easy to realize reduction of the read gap.
In any configuration, in order to realize high recording density, it is required to reduce not only the read gap but also a plane area of the MR element, i.e., a cross sectional area of the MR element on a cross section parallel to film surfaces of layers configuring the MR element. For example, in order to realize recording density of 1 Tbits/in2, it is desirable to reduce an element size to 25 nm×25 nm or less. Especially, the size reduction of the MR element in the track width direction causes a track pitch of a recording medium to be reduced. However, as the cross sectional area of the MR element is reduced, a resistance of the MR element is increased. When the resistance of the MR element is increased, the deterioration in the high-frequency response characteristic of the MR element and the increase in noise occur, and in turn signal to noise ratio (S/N ratio) is deteriorated. Therefore, it is important to suppress the increase in the resistance of the MR element when an element size is reduced. To achieve this, it is important to reduce a resistance-area (RA) of the MR element; and it is desirable that the RA is 0.3 Ωμm2 or less in order to achieve the recording density over 1 Tbits/in2.
Accordingly, a new configuration of a spacer layer has been discussed which allows to realize a small RA and a large magnetoresistance ratio (hereafter, referred to as the MR ratio). The U.S. Patent Application Publication No. 2008/0170336 discloses a spacer layer having a three layer configuration in which Cu layers are arranged on both sides of a ZnO layer, and a metal such as Au, Ag or the like that is less likely to be oxidized than Zn is added to the ZnO layer. By adding the metal to the ZnO layer, the large MR ratio can be obtained while the RA is maintained small. With an MR element using this spacer layer, the MR ratio of approximately 12-13% can be obtained when the RA is 0.3 Ωμm2 or less.
Normally, there is a variation among RAs of MR elements even in one wafer. When the variation is large, the number of MR elements or magnetic heads that can be produced from one wafer is decreased; therefore a drawback remains that a yield rate deteriorates.
It is an object of the present invention to provide an MR element in which a configuration of a spacer layer is improved so that a large MR ratio is realized while an RA variation is suppressed.